Sudan's Army is apparently preparing to stamp out this revolt, which reportedly involves 20,000 to 30,000 Dinka, Nuer, and Shilluk tribesmen. In two incidents last month, the Army said it killed 255 ''outlaws,'' although diplomats say this figure may be exaggerated.

The risk for the US comes in its planned sale to Sudan of two F-5E jet fighters and 36 V150 amphibious armored cars with mounted machine guns. The sale comes in response to an urgent Sudanese request for military equipment, ostensibly to bolster its defenses against Libya and other potential aggressors. But some US observers worry that the jets and military vehicles would be used to put down a strictly internal revolt.

House Foreign Affairs Committee chairman Dante Fascell of Florida, Africa subcommittee chairman Howard Wolpe of Michigan, and black caucus chairman Julian Dixon of California, all Democrats, have appealed to the Reagan administration to postpone the sale.

''Our objection is less to the equipment than to its use,'' says a staff member of the Africa subcommittee. He notes that Sudanese refugees report that F-5Fs (an earlier version of the F-5E) previously sold to Sudan have strafed southern tribesmen with cannon fire.

Providing new fighters and armored cars would ''send a wrong message'' to Sudan, this staffer says. ''We don't want them to think we go along with their counterinsurgency.''

Some American diplomats in Sudan are described as concerned that the Reagan administration views the rebel movement building in the marshy south as simply an attempt by communists to destabilize a friendly regime.

In fact, according to local experts on the region, it is the latest chapter in a 500-year-old feud, which last erupted in 1955 in a 17-year war. These rebels - generally pro-Western Christians and animists who accept arms but not ideology from Libya and Ethiopia - resumed rebel activity after Sudan President Nimeiry last year ended 11 years of southern autonomy. Their ranks have steadily increased since last September, when Nimeiry began attempting to impose extreme Islamic penalties on his only partly Muslim nation.

The situation is complicated by the recent discovery of vast, untapped oil reserves under the Sudd and the half-built 217-mile Jonglei Canal through the swamp, which is intended to increase the White Nile's flow.

In the 1970s, the south's Dinkas - purple-black, often 7 feet tall, thin as rails, with long skinny necks and small heads but huge grins - lived in a sort of happy anarchy tending their big herds of cows.

Their world in the swampland was malaria-infested and swarming with flies and mosquitoes, but one could not help but be impressed with the tribesmen's pride, humor, liveliness, their drumming and dancing.

This writer spent two months in the 1970s living with one southern tribe and later visited a Dinka cattle camp in the swamp itself. I found the Dinkas and Nuers practicing a pastoral system of dry-season cattle grazing and wet-season hoe cultivation believed to have prevailed along the entire Nile Valley 17,000 to 7,000 years ago. But schools were opening, wells were dug, and traders, tailors, and butchers appeared.

Until a year ago the future looked promising for these human relics of neolithic times. The Jonglei Canal would open up the region. Western oil companies, after investing $600 million in exploration, had discovered a vast oil deposit running from El Muglad to the Sudd. It will still take a $900 million pipeline to pump it to the Red Sea for export, but Chevron alone has drilled - and capped - more than 40 wells.

The southerners began to realize what development would bring to them. A Dinka, describing the new money values, says: ''Before we didn't want to sell our cows. Now people know more. A cow can be caught by a snake and die. Money cannot die.''

Some 21/2 years ago, Nimeiry divided the south into three administrative regions - Upper Nile, Bahr el Ghazal, and Equatoria - arguing that he was rescuing the other tribes from Dinka dominance. To most of the tribesmen, this was a transparent effort at divide and rule. The ranks of the guerrillas were fed by resentment over what was seen as northern usurpation of their water and oil resources and political rights.

Six months ago, the tribesmen's anger turned into guerrilla attacks on the French engineers excavating Jonglei and on the oil men; most of these workers pulled out of Sudan and all work on the project stopped.

The Islamic law imposed in Khartoum - but not previously in the south - and the amputations and floggings meted out by kangaroo courts since late April further enraged the southerners, several of whom, though not Muslims, had legs or arms chopped off. Some village women were flogged for brewing sorghum beer, Sudan's national drink and an important part of the diet.

The Dinkas, Nuers, and other tribes maintain they do not seek secession but merely want their autonomy restored, a fair share of the oil revenues, and immunity from Islamic law or penalties. They say their basic, unalterable demand is that Khartoum remove all Arab northern troops from the south.

Also joining forces with the guerrillas have been some Muslims from the Ansar religious faith, followers of the Mahdi, the mysterious holy man who conquered Sudan in the late 19th century. The Ansar, though orthodox Muslims, have bitterly opposed Nimeiry since he jailed the Mahdi's Oxford-educated grandson, Sadiq el Mahdi, their leader, last September.

The biggest of many guerrilla groups, Anyanya (Scorpion) Three, operates in the Upper Nile region near the town of Akobo on the Ethiopian border. Many foreigners have been evacuated from Wau, the main Dinka town, and Malakal, a Nuer stronghold. There is speculation the guerrillas may go in to seize these towns once the rains cut off the roads north. The fear is they would then be bombed by Nimeiry's American-equipped Air Force.

In Juba, capital of Sudan's Equatoria, where Nimeiry's government maintains an army camp, airport, and armored cars, people hope to sit out the fighting. Juba's main fear is that someone may blow up the bridge across the Nile, the city's link to Uganda and Kenya.

Unlike the rest of Sudan, Juba is reportedly normal, as safaris and white hunters come and go, beer is on sale, and most aid projects go right ahead.

The United States has doubled aid to Sudan since 1980 - in fact, it gives Sudan $1 of every $4 in US aid going to the African continent. Aid to Sudan in fiscal year 1984 amounted to a little less than $200 million in economic aid and about $45 million in military aid.